Researchers make flexible circuit breakthrough

Researchers at the University of Pennsylvania have published a paper on a method of constructing high-performance electronic circuits on a flexible plastic substrate using cadmium selenide nanocrystals.

Designed as a replacement for current flexible-circuit construction methods that rely on amorphous silicon, the method promises a cheap means of creating highly-flexible and yet high-performance electronic circuitry. 'We have a performance benchmark in amorphous silicon, which is the material that runs the display in your laptop, among other devices,' explained professor Cherie Kagan, a member of the team responsible for the research. 'Here, we show that these cadmium selenide nanocrystal devices can move electrons 22 times faster than in amorphous silicon.'

That's a significant performance boost, but the technology has yet more to offer: the production of amorphous silicon circuitry requires that the material be deposited at a temperature of several hundred degrees Celsius, while cadmium selenide nanocrystals can be deposited at room temperature and annealed with mild heating. That's important for two reasons: it improves the production process, making it faster, safer and cheaper, but it also means that more flexible plastics that would melt at the temperatures required for amorphous silicon construction can be used.

The process used to create the prototype circuits relied on suspending the nanoncrystals in an ink-type fluid and depositing them using spincoating, a method that relies on centrifugal force to create a thin layer of the material on the substrate. However, the team believes that other common production methods, including dipping, spraying or even printing using ink-jet nozzles, should work fine.

The paper details three example circuits created in the lab to test the performance of the nanocrystals: an inverter, an amplifier and a ring oscillator. 'An inverter is the fundamental building block for more complex circuits,' explained doctoral student Yuming Lai, a member of the research team. 'We can also show amplifiers, which amplify the signal amplitude in analogue circuits, and ring oscillators, where 'on' and 'off' signals are properly propagating over multiple stages in digital circuits.'

'And all of these circuits operate with a couple of volts,' Kagan added at the paper's unveiling. 'If you want electronics for portable devices that are going to work with batteries, they have to operate at low voltage or they won’t be useful.'

Flexible electronics are considered to be a major growth area for the industry: as gadgets become smaller and more portable, cramming circuitry into the space becomes more of a challenge. By using flexible circuits printed on thin plastic, it's possible to get more surface area than previously possible and even to extend the circuit across moving parts of the device such as a watch strap or laptop display hinge.